1. Field of the Invention
This invention relates generally to a power failure detector and, more particularly, to a low voltage/power on reset circuit suitable for implementation on a microprocessor chip.
2. Description of the Prior Art
The advantages offered by MOS technology are wellknown; e.g., higher density, greater yield, etc. Thus, smaller MOS device geometries permit a greater number of devices to be produced per unit area or, stated another way, a single MOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "ON/OFF" or "ONE/ZERO" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to microprocessor chip. However, such arrangements generally require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, analog circuits such as differential amplifiers, and comparators are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing processes be employed. Thus, for example, low voltage and power-on reset circuits employing a comparator which is to be included on an MOS microprocessor chip must be fabricated in accordance with MOS processing techniques, and the design of the circuit must be tailored to such processing techniques.
As is well-known, microprocessors are comprised of numerous counters, registers and other digital circuitry, and it should be obvious that such circuitry must be initialized to a predetermined state whenever the supply voltage is below some minimum operational value; i.e., as when power is first turned on and during power failures. Such circuitry has generally been implemented external to the microprocessor chip and is comprised of discrete components such as timers and comparators in the power supply line which detect a low voltage condition. In addition to the disadvantages of this type of arrangement described above, such external circuitry greatly increases system cost, and users of microprocessor chips are reluctant to undergo the additional expense resulting from the necessity of supplying this type of external circuitry. Secondly, prior art arrangements generally do not provide a satisfactory reset signal. That is, reset signals should not only set the microprocessor components to a defined state, but should also allow sufficient time for the microprocessor's clock oscillators to stabilize.